The present invention generally relates to wireless communication devices and, more particularly, to equalization for demodulation in wireless receivers for communication among wireless devices and access points in a local area network (LAN).
Wireless communication devices, for example, devices using radio frequency signal transmission, may be used for wireless LAN communication. Such wireless LAN communication devices may be described as stations or access points. Stations typically may be found in laptop computers, cell phones, portable modems, or personal digital assistants (PDAs), where they are used for communication with a wired LAN through an access point, which may be briefly described as a wireless transmitter/receiver connected into the wired LAN for interfacing the wired LAN to the wireless communication devices. Stations may also communicate with other stations in a peer-to-peer network, without the presence of an access point, described as operating in “ad-hoc” mode. Such wireless LAN communication devices may be manufactured according to a standard specification in order to secure advantages of standardization such as compatibility between systems made by different manufacturers. One such standard for wireless LAN communication networks, for example, is the 802.11b standard published by the Institute of Electrical and Electronics Engineers, Inc. (IEEE) and, in particular, IEEE Std 802.11b-1999, “Part 11:Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: Higher-Speed Physical Layer Extension in the 2.4 GHz Band”, which is incorporated herein by reference.
A receiver may receive a data transmission for processing, such as according to the 802.11b standard, at rates 1, 2, 3, or 4. The data transmission may originate from a transmitter of another wireless communication device in a wireless LAN according to the 802.11b standard. The data transmission may undergo front-end processing in the receiver, which may include, for example, automatic gain control (AGC), offset control, time-tracking loop (TTL) processing, phase locked loop (PLL) processing, Barker despreading and respreading, estimation of channel impulse response (CIR), and channel matched filter (CMF) processing, as known in the art. Rate 1 and 2 data may undergo code matched filter (CMF) processing and differentially encoded quadrature phase shift keying (DEQPSK) demodulation, as known in the art. Rate 3 and 4 data which, according to the 802.11b standard, is complementary code keying (CCK) data, requires equalization and so may be passed to an equalizer after front-end processing and then to a demodulator for CCK DEQPSK demodulation.
The equalizer may be a decision feedback equalizer (DFE) which typically incorporates two finite impulse response (FIR) filters, one a feedforward filter (FFF) and the other a feedback filter (FBF). The decision feedback equalizer may be used, for example, to suppress intersymbol interference (ISI) caused by dispersive propagation channels. The feedforward filter may operate directly on the data and the feedback filter may operate on previously detected data symbols. The decision feedback equalizer generally outperforms linear equalizers, but degradation in DFE performance, with respect, for example, to suppressing ISI, may occur when incorrectly detected symbols are fed through the feedback filter. Decision errors in feedback to the equalizer may tend to cause yet more incorrect decisions so that decision errors may occur in bursts with a corresponding increase in the average probability of bit and symbol error, as known in the art.
As can be seen, there is a need for using the CCK encoding of symbols in the data supplied to a decision feedback equalizer to reduce decision errors in the decision feedback equalizer. There is also a need for a decision feedback equalizer that takes advantage of the CCK encoding of data to improve the performance of the decision feedback equalizer and DEQPSK demodulation.